Velocity gradients and anelasticity from crustal body wave amplitudes

The amplitude spectra of critically refracted waves are sensitive to small velocity gradients in the refracting horizon. Negative gradients and anelasticity (Q−1) result in similar amplitude decays with distance for narrow bandwidth data; positive gradients result in a net amplitude gain with distance with respect to the zero gradient, infinite Q case. Application of these results to published Pg and P* amplitude data indicates that negative velocity gradients of the order of 10−3 km sec−1 km−1 may be present in the crystalline crust and intermediate layer in the high heat flow Basin and Range province, and that positive velocity gradients are present in corresponding horizons in the provinces having normal heat flow in the eastern United States and west coast. Velocity gradients inferred from laboratory measurements of velocity variations in granites and basic igneous rocks with temperature and pressure, together with published geothermal gradients for the Basin and Range province and the eastern United States, are consistent with this result.

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